Case hardened orthopedic implant

ABSTRACT

A cobalt chrome or austenitic stainless steel alloy orthopedic prosthetic implant. The implant comprises a core and a hardened surface layer. The surface layer is carburized in a process that prevents the formation of carbides on the surface layer.

BACKGROUND

1. Field of the Art

The present invention is related generally to metal orthopedic implantfor use in joint arthroplasty. More specifically, the present inventionis related to case hardened orthopedic implants.

2. Description of the Related Art

Case hardening is a well known manufacturing technique that is used tosurface hardened metal components. The case hardening process involvesmaking the surface layer of a metal component harder by adding a carbonlayer (“carburizing”) or other materials to such surface. The surfacethus becomes very hard and generally comprises higher residualcompressive stresses. An advantage of case hardening a metal componentis that the inner material (“core”) of a case hardened device retainsthe mechanical properties inherent in the material, such a ductility.

Case hardening through carburizing has traditionally been used onferrous-based alloys. The process is also applicable to stainlesssteels, but not without difficulty. First, the chromium oxide surfacethat provides a stainless steel with its corrosion resistant propertiesinhibits the penetration of carbon into the surface of the stainlesssteel. Second, the carburization process is normally carried out at ahigh temperature; however, when stainless steels are carburized at hightemperatures the added carbon forms chromium-carbides in the surfacelayer which decrease or eliminate the corrosion resistant properties ofthe stainless steel.

In the field of orthopedic implant components is necessary to usebiocompatible materials in the construction of prosthetic implantcomponents. Exemplary materials include titanium alloy and cobalt chromealloy. It is often necessary, for the various components used in a jointarthroplasty to articulate against one another. For example, thegenerally metal distal femoral component of a total knee arthroplastyarticulates against the generally polymer bearing component that isinserted between the distal femur and proximal tibia. However, in someorthopedic joint implant designs it is desirable to have two metalcomponents articulate against one another. In these instances, theamount of wear that the surface of such components experiences affectsthe longevity of the implant.

A need exists, therefore, to provide a case hardened cobalt chrome alloyor titanium alloy component.

A further need exists to provide an orthopedic implant comprising ahardened surface and an unaltered core.

A still further need exists to provide orthopedic joint implant design,wherein two case hardened metal joint components articulate against oneanother.

SUMMARY

In one exemplary embodiment, the present invention comprises anorthopedic prosthetic implant component consisting essentially of cobaltchrome alloy, such that the implant comprises a core and surface layerthat includes an articular surface. The surface layer is hardened by acarburization process that discourages the formation of carbides on thesurface layer of the implant component.

An advantage of the present invention is that it provides biocompatibleimplant components having a more wear resistant articular surface, whilemaintaining desirable material properties such as ductility at the coreof the implant component.

Other advantages and features of the present invention will be apparentto those skilled in the art upon a review of the appended claims anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and objects of this invention,and the manner of obtaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescription of an embodiment of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a cross sectional view of a femoral head implant componentaccording to the present invention.

FIG. 2 is a cross sectional view of a femoral stem implant componentaccording to the present invention.

FIG. 3 is a cross sectional view of an acetabular cup implant componentaccording to the present invention.

FIG. 4 is a side elevation view of an acetabular cup implant componentand a femoral head implant component according to the present invention.

Corresponding reference characters indicate corresponding partsthroughout the several views. Although the drawings represent anexemplary embodiment of the present invention, the drawings are notnecessarily to scale and certain features may be exaggerated to betterillustrate and explain the invention. The exemplification set out hereinillustrates an exemplary embodiment of the invention only and suchexemplification should not be seen as limiting of the invention claimedherein.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Cobalt chrome alloy has certain material properties such as strength,ductility, biocompatibility and surface hardness that are useful invarious orthopedic applications. An example of such an application, isprosthetic implants for use with a joint arthroplasty. Jointarthroplasty generally requires two or more prosthetic components tomove against one another. Most orthopedic prosthetic joint designscomprise a plastic bearing component against which one or more metalcomponents move (articulate). In some prosthetic joint designs, however,two or more all metal prosthetic joint components articulate against oneanother. For example, a femoral component, as shown in FIG. 1, mayarticulate against an acetabular cup component (FIG. 3.), as shown inFIG. 4. In prosthetic joint designs, wherein one metal componentarticulates against another metal component it is desirable to increasethe hardness of the articulating surfaces in order to increase suchsurfaces' resistance to wear, pitting, and other physical degradation.An advantage of such metal on metal designs is that articulating metalcomponents, having case hardened articular surfaces, last longer thantraditional polymer bearing components. In a addition, the cores of casehardened metal parts are unaffected by the process thus maintaining theother material qualities of the base metal such as ductility.

The process used to harden the surface of femoral head component 10 isreferred to herein as case hardening. The process comprises activatingthe surface (de-passivation) by applying a layer of iron thereto orexposing the article surface to a hydrogen halide gas mixture ofhydrogen chloride or hydrogen fluoride and nitrogen. Thereafter, thesurface is carburized at a temperature and for a time insufficient forcarbides to form on the surface layer. The details of the process, as itis used for various stainless steels, are described in U.S. Pat. No.6,093,303 (the “303 patent”); the entire disclosure of which is herebyincorporated herein. The 303 patent, however, does not describe using acase hardening process on cobalt chrome alloy or on prosthetic jointcomponents.

Referring now to FIG. 1, there is shown a cross sectional view of afemoral head implant component according to an exemplary embodiment ofthe present invention. The component shown can be used as a prostheticdevice in a typical or minimally invasive hip arthroplasty. As shown inFIG. 1, femoral head component 10 comprises core 110, and case 100disposed therearound. Femoral head component 10 is generally spherical,and further comprises the female portion 120 of a Morse taper styleconnection. Femoral head component 10 further comprises a cobalt chromealloy material. Biocompatible cobalt alloys include: ASTM F-75, ASTMF-1537 (Alloy 1, 2, or 3), ASTM F-90, ASTM F-563, ASTM F-1058 (Grade 1,or 2), ASTM F-562, or ASTM F-799.

Referring again to FIG. 1, there is shown femoral head component 10comprising core 110, and case 100 disposed therearound. Case 100 is arelatively thin layer on the surface of femoral head component 10 thathas been case hardened to increase the surface hardness in comparison tothe base metal.

Referring now to FIG. 3, there is shown a cross sectional view of anacetabular component 30 according to the present invention. Acetabularcup component 30 has a generally concave interior 300 and a convexexterior 310. Acetabular cup component 30 further comprises abiocompatible cobalt chrome alloy material. The interior surface ofacetabular cup component 30 is case hardened to increase the surfacehardness in comparison to the base metal.

As shown in FIG. 4, femoral component 10 is placed in articularengagement with acetabular cup component 30, such that the case hardenedsurfaces of each component are in contact, thereby reducing the wearexperienced by the case hardened surfaces, while maintaining the otherphysical characteristics of the base metal used to construct the implantcomponents.

Although various embodiments of the present invention are described hereand shown in the attached Figures, it is to be understood that thepresent invention may be used with any cobalt chrome orthopedic implant,including, by way of example but not limitation, a patellar implant, aproximal tibial implant for a knee joint, or a distal femoral implantfor a knee joint. The present invention could also be used for non jointrelated orthopedic applications, where it is desirable to have a hardimplant, such as for fracture fixation plates.

1-20. (canceled)
 21. A method for case hardening a surface layer of acomponent consisting essentially of a cobalt chrome alloy, comprisingthe steps of: activating the surface layer of the component by applyinga layer of iron thereto; and carburizing the activated surface layer ofthe component at a temperature and for a time insufficient for carbidesto form on the surface layer.
 22. A method for case hardening a surfacelayer of a component consisting essentially of a cobalt chrome alloy,comprising the steps of: activating the surface layer of the componentby exposing the surface layer to a gaseous hydrogen halide mixture; andcarburizing the activated surface layer of the component at atemperature and for a time insufficient for carbides to form on thesurface layer.